Lead zirconate titanate containing lead is a representative piezoelectric material, and is used in a variety of piezoelectric devices such as an actuator, an oscillator, a sensor, and a filter. However, it has been pointed out that, when a piezoelectric device containing lead is once scrapped and exposed to acid rain, the lead content in the piezoelectric material may be transferred into the soil to adversely affect the ecosystem. Accordingly, in order to exclude lead from piezoelectric devices, research and development on lead-free piezoelectric materials are actively conducted.
In Non Patent Literature 1, it has been found that, by forming a solid solution of a small amount of barium titanate and sodium niobate which is an antiferroelectric, a ferroelectric is produced. Non Patent Literature 1 discloses remanent polarization, coercive field, piezoelectric constants, and electromechanical coupling coefficients of the solid solution in compositions in which the concentration of barium titanate is from 5% to 20%. This material does not contain lead, and in addition, does not contain potassium which is responsible for the difficulty in sintering and reduction in resistance to humidity. Further, it is also disclosed that the Curie temperature of this material is higher than the Curie temperature of barium titanate which is a representative lead-free ferroelectric material (120° C.). The Curie temperature of a composition (Na0.9Ba0.1)(Nb0.9Ti0.1)O3 which may obtain the maximum piezoelectric constant d33=143 pC/N is 230° C.
Patent Literature 1 discloses that, through the addition of cobalt to a solid solution of sodium niobate and barium titanate, the electromechanical coupling coefficient is improved and high heat resistance is obtained. On the other hand, Patent Literature 1 also discloses that such a sample was included that a polarization treatment was difficult because the insulation property was as low as 106Ω or less.